1. Field of the Invention
The present invention relates to a pitch extracting apparatus for extracting a pitch (tone pitch) of the waveform of an input waveform signal.
2. Description of the Related Art
There have been strong demands for extraction of a pitch frequency or pitch period (to be simply referred to as a pitch hereinafter) of a musical tone signal consisting of a repetitive waveform, e.g., a singing voice of a person or a musical tone picked up by a microphone. If a pitch can be extracted, the extracted pitch data can be used for various types of processing. For example, a musical tone corresponding to the pitch (tone pitch) can be generated by an electronic musical instrument or the like.
Various methods have been proposed to extract pitches from audio signals. Of such conventional methods, a method of directly extracting a pitch from an audio signal in a time zone requires only a small amount of arithmetic operations and hence easily allows real-time processing.
According to a first conventional method of extracting a pitch in a time zone, the time interval between adjacent zero-crossing points (time positions at which the amplitude becomes zero) of an input waveform is obtained as the pitch period of the waveform.
According to a second conventional method, a zero-crossing interval is extracted after an input waveform is filtered by a low-pass filter to remove its harmonic components (see, e.g., U.S. Pat. No. 4,688,464 and U.S. Pat. No. 5,018,428).
In addition, according to a third conventional method, the auto-correlation values of an input waveform are calculated, and a pitch period is extracted on the basis of an auto-correlation deviation as a peak value (see, e.g., U.S. Pat. No. 4,633,748).
A human voice, however, contains a considerable amount of harmonic components, and hence a zero-crossing point appears several times within one pitch period of an input waveform. For this reason, if a zero-crossing interval is simply extracted as in the first conventional method, a correct period cannot be obtained, resulting in a large pitch extraction error.
In the second conventional method, if the cutoff frequency of the low-pass filter is decreased, the range in which a pitch can be extracted is limited to a small range of pitch. In contrast to this, if the cutoff frequency is increased, harmonic components are left to increase the pitch extraction error.
Furthermore, in the third conventional method, an enormous amount of arithmetic operations is required in spite of processing in a time zone. If, therefore, arithmetic operations are to be executed with high precision, the pitch of an input waveform cannot be extracted in real-time.